The use of disposable products made from webs of paper, such as paper towels, facial tissues, and sanitary tissues, is pervasive in modern industrialized societies. These products must exhibit certain physical characteristics, such as strength, softness, and absorbency. Strength is the ability of a paper to retain its physical integrity during use. Softness is the pleasing tactile sensation the user perceives when contacting or crumpling the paper. Absorbency is the characteristic of the paper that allows it to take up and retain certain fluids, particularly water, aqueous solutions, and aqueous suspensions. Absorbency encompasses the absolute quantity of fluid a given amount of paper will hold, the rate at which the paper will absorb the fluid and, when the paper is formed into an article (e.g., towel, wipe), the ability of the paper to cause a fluid to preferentially be taken up into the paper and leave a wiped surface dry.
The wet strength of paper can be increased through the incorporation of an additive that both adheres to the pulp, and forms a network that represses swelling of cellulose fibers, inhibiting the separation of fiber-fiber contacts when paper is rewetted. These wet strength additives are typically water soluble, cationic polymers that can form crosslinked networks with themselves or with cellulose. The earliest cationic wet strength resins were condensation products of urea and formaldehyde with small amounts of polyamines.
Polyaminoamide epichlorohydrin (PAE) resins were developed as wet strength resins in the 1950s and 1960s, as described in U.S. Pat. No. 2,926,154, incorporated herein by reference. PAE resins are produced by condensing a polyamine (e.g., diethylenetriamine) with a dibasic acid (e.g., adipic acid) or its ester to form a polyaminoamide. Epichlorohydrin is reacted with the primary and secondary amino groups of the polyaminoamide to form epoxides and chlorohydrins. At neutral pH and above ambient temperatures, the chlorohydrin groups cyclize spontaneously to form 3-hydroxyazetidinium groups. These strained rings confer both reactivity and pH independent cationic charge to the resin macromolecule. Some of the azetidinium groups crosslink the macromolecules concurrently with the alkylation and cyclization reactions during resin manufacture, as described in Espy, Tappi J. 78(4):90-99. Examples of PAE resins are shown below.

PAE with epoxide

PAE with chlorohydrin

PAE with 3-hydroxyazetidinium
An example of the crosslinking mechanism of a PAE resin is shown below.

The materials used to produce PAE resins (e.g., diethylenetriamine, adipic acid, epichlorohydrin) are derived from non-renewable resources, such as petroleum, natural gas, and coal. As used herein, “renewable resource” refers to one that is produced by a natural process at a rate comparable to its rate of consumption (e.g., within a 100 year time frame). The resource can be replenished naturally, or via agricultural techniques. Nonlimiting examples of renewable resources include plants (e.g., sugar cane, beets, corn, potatoes, citrus fruit, woody plants, lignocellulosics, hemicellulosics, cellulosic waste), animals, fish, bacteria, fungi, and forestry products. These resources can be naturally occurring, hybrids, or genetically engineered organisms. Natural resources such as crude oil, natural gas, coal, and peat, which take longer than 100 years to form, are examples of non-renewable resources. As used herein, “petroleum” refers to crude oil and its components of paraffinic, cycloparaffinic, and aromatic hydrocarbons. Crude oil may be obtained from tar sands, bitumen fields, and oil shale.
Thus, the price and availability of the petroleum, natural gas, and coal feedstock ultimately have a significant impact on the price of PAE resins. As the worldwide price of petroleum, natural gas, and/or coal escalates, so does the price of PAE resins and articles made using PAE resins, such as paper towels. Furthermore, many consumers display an aversion to purchasing products that are derived from petrochemicals. In some instances, consumers are hesitant to purchase products made from limited non-renewable resources (e.g., petroleum, natural gas, and coal). Other consumers may have adverse perceptions about products derived from petrochemicals as being “unnatural” or not environmentally friendly.
Accordingly, it would be desirable to provide cationic polymers suitable for use as wet strength resins using monomers derived from renewable resources, where the resulting polymer has desired performance characteristics, such as appropriate wet strength, dry strength, and wet strength to dry strength ratio with no negative impact on properties, such as paper softness.